Terbium type paramagnetic garnet single crystal and magneto-optical device

ABSTRACT

The terbium type paramagnetic garnet single crystal with such characteristics as a high Faraday effect and a high light transmission factor even in a visible range has a high Verdet constant. The magneto-optical device contains the terbium type paramagnetic garnet single crystal. The terbium type paramagnetic garnet single crystal contains at least terbium, at least one element of aluminum and gallium, a part of the terbium being replaced by at least one element of cerium and praseodymium.

TECHNICAL FIELD

The present invention relates to a terbium type paramagnetic garnetsingle crystal having a garnet structure containing at least terbium andat least one element of Al and Ga, which is suitable for use in anoptical isolator and an optical circulator in optical communication andoptical recording and for use in a magneto-optical sensor for detectionof large current.

BACKGROUND ART

In the recent electric power field, great attention has been paid tomagneto-optical sensors which contain magneto-optical materials, and aresuitable for detecting abnormal currents in power transmission lineswhich may be caused by thunder-bolts or the like. The sensors can detectmagnetic fields generated around power transmission lines utilizing theFaraday effect which is a kind of magneto-optical effect. The detectionis carried out utilizing the fact that the Faraday rotational angle ischanged depending on the intensity of a magnetic field. It is generallyknown that when a magneto-optical element having the Faraday effect isirradiated with a laser beam, and a magnetic field is generated in thesame direction of the propagation direction of the laser beam, thepolarization plane of the incident laser beam is rotated in proportionto the intensity of the magnetic field. Polarizing plates havingdifferent polarization planes are arranged on the front and back sidesin the light propagation direction of the magneto-optical element,utilizing the above-described rotation of the polarization plane.Accordingly, the difference between the rotational angles of thepolarization planes causes the difference between the light-quantitiesto appear. The difference between the light-quantities is detected by alight-sensing means such as a photodetector or the like. Thus, thestrength of abnormal current can be detected. Magneto-optical sensorsusing the Faraday effect as described above have a high sensitivity.Moreover, the sizes and weights can be reduced. Furthermore, theexplosion-proof performance is high, and the sensors can beremote-controlled. Also, since optical fibers are used for propagationof light. Thus, the electromagnetic induction noise levels and theinsulating properties are superior. Thus, the magneto-optical sensorshave such superior characteristics as cannot be obtained by electricaltype magnetic field sensors.

Referring to the characteristics of paramagnetic materials to formmagneto-optical elements, it has been required that the Verdet constant(V: (deg/(Oe·cm)) is high. The Verdet constant means a Faradayrotational angle per unit length and per unit applied magnetic field.The Verdet constant has a relationship represented by θ_(f)=VHd, inwhich θ_(f) represents a Faraday rotational angle meaning the angle of apolarized light beam, d represents a movement distance of the light beamwhich passes through the magneto-optical element, and H represents theintensity of a magnetic field applied to the magneto-optical element.Thus, the change ratio of the Faraday rotational angle increases as theVerdet constant becomes larger. Thus, the difference between thelight-quantities is ready to increase with the magnetic field beingslightly changed. Thus, a magneto-optical sensor having a highsensitivity can be provided.

As a magnetic material having the above-described properties, a singlecrystal having an yttrium iron garnet structure (Y₃Fe₅O₁₂: hereinafter,referred to as YIG for short) which is a ferromagnetic material asdescribed in Japanese Examined Patent Application Publication No.2-3173. The YIG single crystal is advantageous in that the Verdetconstant is large, and the sensitivity to magnetic variation is high.However, the YIG single crystal described in Japanese Examined PatentApplication Publication No. 2-3173 has the following problems: theFaraday rotational angle increases till the intensity of a magneticfield reaches a predetermined value, and becomes constant after theintensity reaches the predetermined value, i.e., the Faraday rotationalangle becomes magnetically saturated. Therefore, when the YIG singlecrystal is used as a magneto-optical element of a magneto-optical sensorfor detection of large current, problems occur in that the sensor cannot accurately detect electric current. Also, the YIG single crystal cantransmit only light rays in an infrared range of 1000 nm to 5000 nm.Thus, problematically, the YIG single crystal cannot be used in avisible range of 400 nm to 600 nm and at a wavelength of 650 nm. Thewavelength of 650 nm is in the wavelength range for plastic fibers whichhave been investigated for use in LAN or the like which is mounted oncars. Light sources for use in an infrared range are expensive. On theother hand, light sources in a visible range are inexpensive. Thus, itis desired to realize a paramagnetic material which can be used in avisible range.

As a magnetic material which can solve the above-described problems, aterbium.aluminum type paramagnetic garnet single crystal (Tb₃Al₅O₁₂;hereinafter, referred to as TAG single crystal for short) containing atleast Tb and Al is described, e.g., in a literature of S. Ganschow, D.Klimm, P. Reiche and R. Uecker; Cryst. Res. Technol., 34 (1999) pp.615-619. The Verdet constant of the TAG single crystal is very largecompared to the Verdet constant constants of other paramagneticmaterials. Thus, even if the size of the single crystal is reduced, asufficient Faraday rotational angle can be obtained. Accordingly, thesizes of magneto-optical elements can be reduced. Furthermore, even if astrong magnetic field is applied, no magnetic saturation occurs incontrast to the YIG single crystal. Therefore, the TAG single crystalcan be used as a magneto-optical element of a magneto-optical device fordetection of large electric current. Hence, the TAG single crystal candetect a wide range of magnetic field intensity. The TAG single crystalhas a very high light transmission factor in a light wavelength range of500 nm to 1400 nm. In addition, it has been revealed that the TAG singlecrystal can be provided with a high light transmission factor in avisible range of 400 nm to 700 nm in wavelength. It is suggested topositively use the TAG single crystal having the above-describedsuperior properties in magneto-optical devices.

Although the TAG single crystal has the above-described superiorproperties, no TAG single crystals with such a large size as to bepractically applied in magneto-optical devices have been realized. Thereason is described below. The TAG single crystal is adecomposition-melting type compound. Thus, the composition of startingraw materials obtained when the materials are melted is different fromthe composition of a crystal obtained when the melted raw materials arecooled. More specifically, the decomposition-melting type TAG singlecrystal composed of a garnet phase cannot be obtained directly from thecomposition of the melted starting raw materials. Thus, problems occurin that TbAlO₃ composed of a perovskite phase is mixed with the TAGsingle crystal. The TAG single crystal has a largest Verdet constant ofthe paramagnetic dielectrics. However, the Verdet constant of the TAGsingle crystal when it is irradiated with a light beam with a wavelengthof 633 nm is about 0.01°/(Oe·cm). Thus, for application of the TAGcrystal for magneto-optical devices, the TAG single crystal is requiredto have a still larger Verdet constant.

As another terbium type paramagnetic garnet single crystal, for example,terbium.gallium.garnet (Tb₃Ga₃O₁₂: hereinafter, referred to as TGGsingle crystal for short) or the like is known. However, the Verdetconstant must be increases as well as that of the TAG single crystal.The TGG single crystal is a coincidently melting type material. That is,the composition of the starting raw materials of the TGG single crystalis the same as that obtained after the crystal is grown. Thus, the TGGsingle crystal having a practical size can be easily produced by theknown Czochralski process. However, the obtained TGG single crystal hasproblems in that the Verdet constant at a wavelength of 633 nm is small,i.e., 0.0075°/(Oe·cm). Magneto-optical materials which have a largeVerdet constant and are effective in size-reduction are desired for thedecomposition-melting type TAG single crystal and also coincidentlymelting type terbium type paramagnetic garnet materials.

It is an object of the present invention to solve the above-describedproblems and to provide a terbium type paramagnetic garnet singlecrystal of which the Faraday effect is large, the light transmissionfactor is high, and the Verdet constant is enhanced, and to provide amagneto-optical device using the terbium type paramagnetic singlecrystal.

DISCLOSURE OF INVENTION

To achieve the above-described object, according to a first invention ofthis application, there is provided a terbium type paramagnetic garnetsingle crystal containing at least terbium and at least one element ofaluminum and gallium, wherein at least one element of cerium andpraseodymium is substituted for a part of the terbium.

Preferably, in the terbium type paramagnetic garnet single crystalaccording to a second invention of this application, the single crystalis represented by the following chemical formula: (Tb_(3-x)M_(x))N₅O₁₂,in which M represents at least one element of Ce and Pr, N represents atleast one of Al and Ga, and x is in the range of 0.01≦x≦2.

Preferably, in the terbium type paramagnetic garnet single crystalaccording to a third invention of this application, the single crystalis represented by the following chemical formula: (Tb_(3-x)M_(x))N₅O₁₂,in which M represents Ce, N represents at least one of Al and Ga, and xis in the range of 0.01≦x≦1.

Preferably, in the terbium type paramagnetic garnet single crystalaccording to a fourth invention of this application, the single crystalis represented by the following chemical formula: (Tb_(3-x)M_(x))N₅O₁₂,in which M represents Pr, N represents at least one of Al and Ga, and xis in the range of 0.01≦x≦2.

Preferably, the magneto-optical device according to a fifth invention ofthis application includes, as a magneto-optical element, the terbiumtype paramagnetic single crystal defined in the first to fourthinventions of this application.

As described above, with the composition according to the firstinvention, a terbium type paramagnetic garnet single crystal having aVerdet constant larger compared to that of the non-substitution type TAGsingle crystal or TGG single crystal can be provided. A small-sizedmagneto-optical device which can transmit even a visible light can beprovided by use of the above-described terbium type paramagnetic garnetsingle crystal.

Moreover, with the compositions according to the second to fourthinventions of this application, a terbium type paramagnetic garnetsingle crystal having a large Verdet constant can be securely provided.Preferably, according to the fourth invention of this application, Pr issubstituted with high stability.

Moreover, according to the fifth invention of this application, themagneto-optical device has a larger Verdet constant, and thus, amagneto-optical device having a larger Faraday effect can be provided.This device can transmit even a visible light with a wavelength range of400 to 650 nm. Thus, a magneto-optical device which copes with a laserhaving a wavelength in a visible range can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a laser FZ apparatus for usein an embodiment of the present invention.

FIG. 2 is a reflection-type X-ray Laue photograph of a sample 4according to the present invention.

FIG. 3 is a reflection-type X-ray Laue photograph of a ample 9 accordingto the present invention.

FIG. 4 is a reflection-type X-ray Laue photograph of a reference sample1.

FIG. 5 schematically shows the structure of a magneto-optical deviceaccording to an embodiment of the present invention.

FIG. 6 shows the magnetic field—time characteristics of sample 2, 7, and10.

FIG. 7 shows the light-quantity—time characteristics of the samples 2,7, and 10.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the terbium type paramagnetic garnet single crystal of thepresent invention and the magneto-optical element using the singlecrystal will be described.

In the terbium type paramagnetic garnet single crystal containing atleast terbium and at least one element of aluminum and gallium of thepresent invention, at least one element of cerium and praseodymium issubstituted for a part of the terbium. Both of cerium and praseodymiummay be added. Preferably, the single crystal is represented by thefollowing chemical formula:(Tb_(3-x)M_(x))N₅O₁₂.

Preferably, at least one element of Ce and Pr is substituted as M, atleast one of Al and Ga is substituted as N. Preferably, x is in therange of 0.01≦x≦2.

In the case where cerium is substituted for a part of the terbium,preferably, x is in the range of 0.01≦x≦1.0 If x is less than 0.01,effects of the substitution of Ce are not sufficient. If x is more than1.0, the Verdet constant becomes saturated and is kept constant, even ifCe is further substituted. The amount of Ce exceeds the solution limitof Ce in the terbium type paramagnetic garnet. Moreover, if an excessamount of Ce is added, the Ce not solid-dissolved segregates in thecrystal, and thus, undesirably, the transmission factor may decrease.

In the case where praseodymium is substituted for a part of the terbium,preferably, x is in the range of 0.01≦x≦2.0. If x is less than 0.01,effects of the substitution of praseodymium are not sufficient.Therefore, undesirably, the Verdet constant does not become large. If xis more than 2.0, the amount of praseodymium exceeds the solution limitof praseodymium in the terbium type paramagnetic garnet. Moreover, if anexcess amount of praseodymium is added, the praseodymium notsolid-dissolved segregates in the crystal, and thus, undesirably, thetransmission factor may decrease. Since the solid-solution limit ofpraseodymium is 2.0, praseodymium can be substituted for a part of theterbium in a larger amount compared to cerium. Thus, a larger Verdetconstant can be obtained. Cerium and praseodymium, when they have avalence number of 3, contribute directly to the Faraday effect. However,cerium is more stable in the form of Ce⁴⁺ than in the form of Ce³⁺.Accordingly, even if it is attempted for cerium to be simplysubstituted, CeO₂ may be deposited. Therefore, in the case where ceriumis substituted, the crystallization and growth must be carried out in areducing atmosphere. On the other hand, when praseodymium issubstituted, the crystallization and growth can be easily carried out,since the praseodymium is stable in the form of Pr³⁺ and is easilysubstituted. Moreover, the crystal can be grown in the atmosphere.Therefore, praseodymium is more preferable than cerium.

Moreover, in the terbium type paramagnetic garnet single crystal of thepresent invention, one element of cerium and praseodymium is substitutedfor a part of the terbium, and moreover, a rare earth element other thancerium and praseodymium may be substituted for a part of the terbium.Preferably, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, or the like isused as the rare earth element. In the case where sun a rare earthelement is added, the terbium type paramagnetic garnet single crystal isrepresented by the following formula: (Tb_(3-x-y)M_(x)R_(y))N₅O₁₂, inwhich M represents at least one element of cerium and praseodymium, andN represents at least one element of aluminum and gallium. Preferably, yis in the range of from 0 to 1. In other words, the rare earth elementdoes not have to be added. Moreover, if y exceeds 1, undesirably, theVerdet constant decreases, since the total number of terbium ionssignificantly contributing to the Faraday effect decreases.

According to the present invention, Fe may be contained as impurities.However, the amount is less than 50 ppm. In some cases, the Fe can notsubstantially be detected, depending on the type of an analyticaldevice.

Referring to an appropriate method of producing the TAG single crystalof the present invention, preferably, a raw material bar has aparamagnetic garnet structure containing at least Tb and Al, and atleast one of the raw material bar and the seed crystal is porous.Preferably, the production is carried out by the laser FZ methodcomprising a first step of preparing the raw material bar and the seedcrystal, a second step of melt-bonding the raw material bar and the seedcrystal to each other by irradiation with light energy, and a third stepof shifting the light energy irradiation area from the melt-bondingportion to the raw material bar side.

A perovskite phase of TbAlO₃ or the like can be eliminated from themelting zone, and the TAG single crystal can be easily formed by theabove-described production method. Thus, such a large TAG single crystalas can not be produced by the known production methods can be easilyproduced. Thereby, the number of chips formed by cutting of the TAGsingle crystal can be increased, and can be positively used as materialsfor magneto-optical devices. According to the known production methods,regarding Tb₃Al₃O₁₂ which is a decomposition melting type, first, aperovskite phase is formed in the solid-liquid interface between themelting zone and the solid, and thereafter, Tb₃Al₅O₁₂ is formed. Thus,inevitably, the garnet phase and the perovskite phase are mixedtogether. For the known FZ method, the raw material bar and the seedcrystal are required to have a high density. To the contrary, it isfound by the inventors that at least one of the raw material bar and theseed crystal is formed so as to be porous, and the raw material bar andthe seed crystal are melt-bonded to each other by application oflight-energy, resulting in formation of a melting zone, wherein themelt-liquid penetrates into the porous medium, and the perovskite phasegenerated as an initial phase precedently deposited in the porousmedium. Thus, the perovskite phase is eliminated from the melting zoneby the deposition of the perovskite phase as an initial phase into theporous medium as described above. Thus, a larger terbium type garnetsingle phase can be obtained.

An apparatus for producing a single crystal which is suitable forrealization of the above-described production method will be described.FIG. 1 is a schematic perspective view of a laser FZ (Floating Zone)apparatus which is used according to a first embodiment of the presentinvention.

The laser FZ apparatus 1 comprises laser devices 5 which generate laserbeams, a box 3 which also functions as a heat-radiating plate, and ashaft 2 capable of being vertically moved in the box 3. The shaft 2 is arod-shaped supporting device which can be separated into an upper shaft2 a and a lower shaft 2 b, vertically extends through the upper andlower surfaces of the box 3, and is formed so as to be moved in thevertical direction. Pieces to be supported can be held on the uppershaft 2 a and the lower shaft 2 b, respectively. Windows havingwindow-lenses 4 a fitted therein are provided on both side-faces of thebox 3 in such a manner that the joining portion of the piece supportedon the shaft 2 a and the piece supported on the shaft 2 b can beirradiated with laser beams in concentration. The laser devices 5 aredisposed on both sides of the box 3 in such a manner that laser beamscan be irradiated through both of the windows 4 of the box 3. Laserbeams irradiated by the laser devices 5 positioned on both sides of thebox 3 pass through the window lenses fitted in the windows 4 of the box3. Thus, the pieces supported on the shaft 2 are irradiated with thelaser beams in concentration from the opposite directions.

As described above, the shaft 2 has such a structure that the shaft 2can be moved in the axial direction. Thus, the irradiation area of thelaser beams irradiated to the pieces supported on the shaft can beshifted. With the above-described structure, advantageously, the rawmaterial bar and the seed crystal can be continuously melt-joinedtogether, and also, the obtained melting zone can be continuouslycooled. More specifically, the raw material bar is held on the uppershaft 2 a, and the seed crystal is held on the lower shaft 2 b. The oneend of the raw material bar is irradiated with laser beams, and thus ismelted. The melt-portion is caused to contact one end of the seedcrystal held on the lower shaft 2 b while light energy is applied to themelt-portion, whereby the raw material bar and the seed crystal aremelted and joined to each other to form a melting zone. Then, the shaft2 is moved downward in the axial direction. Thus, a further melting zoneis formed on the raw material bar side of the melt-joined portion of theraw material bar and the seed crystal. Successively, the shaft 2 ismoved downward in the axial direction, so that the melt-liquid isgradually cooled starting on the seed crystal side and is solidified. Itshould be noted that the raw material bar held on the upper shaft 2 aand the seed crystal held on the lower shaft 2 b may be caused tocontact each other, and thereafter, the contact-portion is melted, andthus, the raw material bar and the seed crystal are melt-joined.

Preferably, the movement speed of the shaft is not more than 30 mm/hour,depending on the diameter of a TAG polycrystal used as a raw material.If the movement speed in the axial direction of the melting zone ishigher than 30 mm/hour, the crystal can not be sufficiently melted, andthus, it is difficult to convert the melting-zone to a single crystal.In this case, the irradiation position of the laser beam is fixed, andthe shaft 2 can be shifted in the axial direction. The laser FZapparatus may have such a structure that laser beams can be shifted.

Regarding the laser devices 5 used here, preferably, the wavelength isin the range of from 1.6 μm to 100 μm. It should be noted that awavelength of 100 μm is the maximum wavelength of light. A TAG singlecrystal can not absorb a laser beam of which the wavelength is less than6 μm. Hence, probably, the raw material bar can not sufficiently bemelted. As a laser device with a wavelength of not less than 1.6 μm, aCO₂ gas laser is preferable. In the case of the CO₂ gas laser device,the wavelength of a laser beam irradiated is large. Thus, even if thecrystal is a TAG polycrystal having a wide transparent wavelength zone,the crystal can efficiently absorb the laser beam, and the TAGpolycrystal can be easily melt-joined. In addition, an excimer laserdevice may be employed.

As described above, the two laser devices 5 are disposed on both sidesof the box 3, so that the shaft is irradiated with laser beams from bothsides thereof. However, the positions of the laser devices 5 are notrestricted to both sides of the box 3. Thus, it is necessary to applylight energy to the joined-portion of the TAG polycrystal and the seedcrystal in concentration. At least three laser devices may be arranged,and laser beams are irradiated to the joined-portion from positions onthe normal. Thus, the temperature-gradient can be set steeper, and theirradiation area can be increased by arrangement of plural laser beamoscillation sources as described above. Thus, a larger TAG singlecrystal can be provided.

Also, the intensity of a laser beam can be controlled by adjustment ofthe distances between the laser devices 5 and the shaft 2 on which theparamagnetic garnet single crystal 7 and the seed crystal 8 are held.The laser beams can be appropriately controlled by adjustment of thesize of a crystal to be formed, the size of the box 3, the focaldistances of the lenses 4, and so forth.

Also, according to the present invention, to irradiate the end of a TAGpolycrystal with laser beams in more concentration, preferably, acollective lens member 6 is further provided. More specifically, thecollective lens member 6 is arranged in such a position that acollective lens 6 a of the member 6 exists on the prolonged line passingthrough the windows of the box 3 and the laser oscillation ports 5 a,and laser beams can be irradiated through the windows of the box 3 tothe end of a pieces supported on the upper shaft 2 a. The collectivelenses 6 a are not necessarily the same as the window lenses 4 a of thewindows 4 of the box 3. In the case where a CO₂ gas laser is employed,preferably, the collective lenses 6 a are made of ZnSe. Furthermore,means other than collective lenses may be used, provided that the meanscan collect a laser beam.

In the above-description, as the pieces to be supported, the rawmaterial bar and the seed crystal are used. The raw material bar is heldon the shaft 2 a, and the seed crystal is held on the shaft 2 b. On theother hand, the seed crystal may be held on the shaft 2 a, and the rawmaterial bar may be held on the shaft 2 b. Moreover, the upper shafts 2a and 2 b may be rotated at the same speed and in the same direction, sothat laser beams can be irradiated to a wide area of the joined portionof the piece held on the shaft 2 a and the piece held on the shaft 2 b.

The TAG single crystal is a coincidently melting type compound.Accordingly, the single crystal may be grown by the Czochralski methodor the like, in addition to the above-described laser FZ method.

The TAG single crystal and the TGG single crystal, obtained by theproduction method for the terbium type paramagnetic garnet singlecrystal of the present invention, can be used in magneto-opticaldevices, more specifically, optical isolators, optical attenuators,optical switches, optical circulators, and so forth. In addition, theTAG single crystal and the TGG single crystal can be used as differenttypes of magneto-optical sensor materials for rotation sensors,flow-rate sensors, current sensors, and so forth.

An embodiment of the magneto-optical device of the present inventionwill be described with reference to a magneto-optical sensor shown inFIG. 5. Hereinafter, an embodiment of the magneto-optical sensoraccording to the present invention will be described with reference toFIG. 5. In the below-description, the term “optical axis” has a generalmeaning used for description of a light propagation line. Moreover, theterm “incidence” means a direction in which a light beam irradiated by alight source enters the magneto-optical sensor at the first, and theterm “outgoing” means a direction in which the light beam exits from themagneto-optical sensor. FIG. 5 schematically shows the constitution ofthe magneto-optical sensor of the present invention. The magneto-opticalsensor 11 comprises a Faraday rotor 12, a polarizer 13, an analyzer 14,a light-irradiating means 15, and a light-sensing means 16. In thiscase, the polarizer 13 is disposed on the incident side of the Faradayrotor 12, while the analyzer 14 is arranged in parallel to the opticalaxis on the outgoing side of the Faraday rotor 12. The polarizer 13 andthe analyzer 14 are disposed in such a manner that the polarizationplanes thereof are perpendicular to the optical axis, so that theoptical axis passing through the Faraday rotor 12 can pass through thepolarization planes. The light irradiating means 15 is provided in sucha position that a light beam can be made incident upon the polarizer 13.That is, the light irradiating means 15 is not necessarily disposedalong the optical axis passing through the Faraday rotor 12. Forexample, a total reflecting mirror 17 a may be provided between thelight irradiating means 15 and the polarizer 13. Thus, a light beam isreflected from the total reflecting mirror 17 a, and the optical path ischanged. The magneto-optical sensor 11 can be reduced in size due to thetotal reflecting mirror 17 a provided between the light irradiatingmeans 15 and the polarizer 13. Moreover, a total reflecting mirror 17 bmay be provided between the analyzer 14 and the light-sensing means 16for the same reason as that for the total reflecting mirror 17 a.Moreover, collective lenses 18 a and 18 b may be provided between thetotal reflecting mirror 17 a and the polarizer 13 and between the totalreflecting mirror 17 b and the analyzer 14, respectively. In this case,a light beam can be efficiently made incident on the Faraday rotor 12due to the collective lenses 18 a provided. Moreover, a light beam canbe efficiently made on the light sensing means 16 due to the collectivelens 18 b provided. Preferably, the terbium type paramagnetic garnetsingle crystal of the present invention can be used as the Faraday rotor12 of the above-described magneto-optical sensor.

It is assumed that an impeller having permanent magnets attached thereonis arranged on the side of the Faraday rotor to generate a magneticfield, and the magnetic field is applied in parallel to the optical axisof the Faraday rotor. Based on the assumption, the action of theabove-described magneto-optical sensor will be described below. First, alight beam irradiated by the light irradiating means 15 reflects on thetotal reflecting mirror 17 a at an angle of 90° in such a manner thatthe reflected light beam becomes parallel to the optical axis passingthrough the Faraday rotor 12. The reflected light beam is colleted bythe collective lens 18 b, and passes through the polarizer 13. Thus, thelight beam having the same polarization plane as the polarizer 13 isincident upon the Faraday rotor 12. In this case, the polarization planeof the incident light beam is rotated by an amount corresponding to theFaraday rotational angle with respect to the propagation direction ofthe light beam in the Faraday rotor 12, due to the Faraday effect. Thus,the light beam outgoing from the Faraday rotor 12 passes through theanalyzer 14, so that the light beam having the same polarization planeas the analyzer 14 can be output. The light beam outgoing from theanalyzer 14 passes through the collective lens 18 b, and thereafter, thelight beam is reflected on the total reflecting mirror 17 b at an angleof 90° to the optical axis. Regarding the reflected light beam, thelight-quantity is detected by the light-sensing means 16.

[Experiment 1]

EXAMPLE 1

1. First Process

First, as starting raw materials for paramagnetic garnet polycrystals,Tb₄O₇ (purity: 99.9%), Al₂O₃ (purity: 99.99%), and Pr₂O₃ (purity: 99.9%)were prepared in such a manner that the grown crystals had thecompositions of samples 1 to 5 shown in Table 1. Then, toluene and adispersant were added to the mixed powder of the starting raw materials.The mixed powder was mixed and crushed for about 48 hours by means ofballs. An organic binder was added to the formed liquid-mixture, andfurther mixed for 24 hours. Thus, the obtained slurry-like mixture wasde-aired, and formed into a sheet with a thickness of about 50 μm bymeans of a squeegee. Plural raw material sheets formed as describedabove were laminated, and press-bonded together by means of ahydrostatic press. Then, the laminate was cut into rectangles. Thusobtained laminate chips were fired at 1600° C. for 2 hours. Thus, TAGtype polycrystals for the samples 1 to 5, having a rod shape, wasproduced. The sintering density of the TAG polycrystals was 80%.

2. Second Process

Each of the TAG type polycrystals 7 for the samples 1 to 5, produced inthe first process, was placed in the above-described laser FZ apparatus1 in such a manner that each TAG type polycrystal was held on one end ofthe upper shaft 2 a of the laser FZ apparatus. A seed crystal 8consisting of a TAG single crystal was placed on one end of the lowershaft 2 b. The inside of the box 3 was under the atmosphere. Laser beamswere irradiated by the laser devices 5, so that the end of the TAGpolycrystal 7 disposed on the one-end of the upper shaft 2 a was heatedand thus melted. The melted part of the TAG polycrystal 7 was joined tothe one-end of the seed crystal 8 disposed on the lower shaft 2 b.

3. Third Process

Subsequently, the melt-joined-portion of the end of each of the TAGpolycrystals 7 for the samples 1 to 5 and the end of the seed crystal 8was irradiated with laser beams. Thus, a melting zone was formed. Inthis case, the distance between the laser devices and the melting zonewas about 50 cm. More specifically, the laser devices used here were CO₂gas laser devices which can output a CO₂ gas laser beam with awavelength of 10.6 μm. The output was 60 W.

Subsequently, the shaft was moved downward in the axial direction at aspeed of 30 mm/hour or lower. Thereby, the laser-beam irradiation areawas shifted toward the raw material bar side of the melt-joined portion.The melt-liquid existing on the seed crystal side in the melting zonewas cooled and thus solidified. The obtained crystals were taken as thesamples 1 to 5. The shapes of the obtained crystals were determined. Thecrystals were fibrous, having a diameter of 1 mm and a length of 27 mm.

EXAMPLE 2

Crystals were produced in the same manner as that of Example 1 exceptthat as starting raw materials for paramagnetic garnet polycrystals,Tb₄O₇ (purity: 99.9%), Al₂O₃ (purity: 99.99%), and CeO₂ (purity: 99.9%)were prepared so that the grown crystals had the compositions of thesamples 6 to 9 shown in Table 1, and the crystals were grown in anreducing atmosphere.

COMPARATIVE EXAMPLE 1

A crystal was produced in the same manner as that of Example 1 exceptthat as starting raw materials for a paramagnetic garnet polycrystal,Tb₄O₇ (purity: 99.9%) and Al₂O₃ (purity: 99.99%) were prepared, and apure TAG single crystal having the crystal composition of the sample 10shown in Table 1 was produced.

COMPARATIVE EXAMPLE 2

Crystals were produced in the same manner as that of Example 1 exceptthat as starting raw materials for paramagnetic garnet polycrystals,Tb₄O₇ (purity: 99.9%), Al₂O₃ (purity: 99.99%), Nd₂O₃ (purity: 99.9%),Sm₂O₃ (purity: 99.9%), and Eu₂O₃ (purity: 99.9%) were prepared, and thecrystals had the crystal compositions of the TAG type polycrystals ofthe samples 11 to 13 shown in Table 1.

It was evaluated whether the crystals of the samples 1 to 13 produced asdescribed above were the object single crystals or not according to themethod described below.

Regarding the sample 4 obtained in Example 1, the sample 9 obtained inExample 2, and the pure TAG single crystal of the sample 10 obtained inComparative Example 1, the reflection type X-ray Laue photographs weretaken. FIGS. 2 to 4 show the photographs of the samples 4, 5, and 10 inthat order. As seen in FIGS. 2 to 4, the reflection type Laue images ofthe TAG single crystals are coincident with each other, even in the casewhere Pr or Ce is substituted for Tb sites. Thus, it is seen that thecontrol of a growing-direction could be performed. Thus, it is seen thatthe samples 4 and 9 produced according to the production method of thepresent invention are terbium.aluminum paramagnetic garnet singlecrystals.

Then, the crystals of the samples 1 to 13 were cut into a disc shape,and mirror-finished by polishing with a high-precision polishing device.The Faraday rotor 2 was irradiated with He—Ne laser beams with an outputof 1 mW and a wavelength of 633 nm while a magnetic field (H) of 1 kOewas applied. Then, regarding the light beam outgoing from the Faradayrotor, the Faraday rotation angle (θ_(f)) was determined by the CrossNicol method. The value was substituted for θ_(f) of V=θ_(f)/(d×H).Thus, the effective Verdet constant was determined. Table 1 shows theresults. TABLE 1 Sample Crystal Verdet constant number composition (°/Oe· cm) *1 Tb_(2.991)Pr_(0.009)Al₅O₁₂ 0.0097 2 Tb_(2.8)Pr_(0.2)Al₅O₁₂0.0140 3 Tb_(2.5)Pr_(0.5)Al₅O₁₂ 0.0350 4 Tb₂Pr₁Al₅O₁₂ 0.0700 5Tb₁Pr₂Al₅O₁₂ 0.1400 *6 Tb_(2.991)Ce_(0.009)Al₅O₁₂ 0.0100 7Tb_(2.8)Ce_(0.2)Al₅O₁₂ 0.0160 8 Tb_(2.5)Ce_(0.5)Al₅O₁₂ 0.0400 9Tb₂Ce₁Al₅O₁₂ 0.0800 *10 Tb₃Al₅O₁₂ 0.0092 *11 Tb_(2.8)Nd_(0.2)Al₅O₁₂0.0090 *12 Tb_(2.8)Sm_(0.2)Al₅O₁₂ — *13 Tb_(2.8)Eu_(0.2)Al₅O₁₂ 0.0079

As seen in Table 1, the Verdet constants of the samples 11 to 13 inwhich at least one of cerium and praseodymium is not contained, and theterbium is replaced by another additive are smaller compared to theVerdet constant of the sample 10 which is a pure TAG single crystal. Onthe other hand, the Verdet constants of the samples 2 to 5 and thesamples 7 to 9, which are within the scope of the present invention, arelarger compared to the Verdet constant of the sample 10 which is a pureTAG single crystal. Regarding the samples 1 to 6 in which thecomposition ratios by number of cerium and praseodymium are less than0.01, it is seen that the effects of the cerium and the praseodymium arenot so sufficient to increase the Verdet constants. The sample 12absorbed a He—Ne laser beam with a wavelength of 633 μm, and thus, thedetermination was impossible.

[Experiment 2]

EXAMPLE 3

First, as starting raw materials for paramagnetic garnet, Tb₄O₇ (purity:99.9%), Ga₂O₃ (purity: 99.99%), and Pr₂O₃ (purity: 99.9%) were preparedin such a manner that the crystals had the compositions of the samples14 to 18 shown in Table 1. The prepared raw materials, together withballs, were placed into a plastic container, and dry-mixed for one hour.The balls and the raw materials in the plastic container were placedinto a mesh basket, so that only the balls were removed. The rawmaterials were placed into an Ir-made crucible, and was calcined at atemperature of 1200° C. for 2 hours in the atmosphere in an electricalfurnace. The calcined powder was sieved through a mesh-sieve and chargedinto a rubber mold. The powder was pressed with a hydrostatic press at apressure of 2000 kgf/cm², and in this state, was let to stand for 1minute. Thus, the bulk density of the raw materials was increased. Thepressed raw materials were charged into an Ir-made crucible, placed inthe chamber of a high frequency induction heating device. The cruciblewas high-frequency induction-heated, and thus, the temperatures of theraw materials rose. In this case, the inside of the chamber was setunder the atmosphere for the crystal-growth. After it was confirmed thatthe raw materials in the crucible were melted, the melt-liquid was madeto contact the TGG single crystal as a seed crystal. After themelt-liquid and the TGG single crystal were kept in sufficient contactwith each other, the melt-liquid was lifted at a speed of 1 mm/h whileit was rotated at a rotational speed of 4 rpm. Thus, the TGG singlecrystals of the samples 14 to 18 were produced. The obtained crystalshad a bulk shape with a diameter of 50 mm and a length of 100 mm.

EXAMPLE 4

Crystals were produced in the same manner as that of Example 3 exceptthat as raw materials for paramagnetic garnet polycrystals, Tb₄O₇(purity: 99.9%), Ga₂O₃ (purity: 99.99%), and CeO₂ (purity: 99.9%) wereprepared, the crystals had the compositions of the samples 19 to 23shown in Table 1, and the crystals were grown in an reducing atmosphere.

COMPARATIVE EXAMPLE 3

A crystal was produced in the same manner as that of Example 3 exceptthat as raw materials for a paramagnetic garnet polycrystal, Tb₄O₇(purity: 99.9%) and Ga₂O₃ (purity: 99.99%) were prepared, and a pure TAGsingle crystal having the crystal composition of the sample 24 shown inTable 1 was produced.

COMPARATIVE EXAMPLE 4

Crystals were produced in the same manner as that of Example 3 exceptthat as raw materials for paramagnetic garnet polycrystals, Tb₄O₇(purity: 99.9%) Ga₂O₃ (purity: 99.99%), Nd₂O₃ (purity: 99.9%), Sm₂O₃(purity: 99.9%), and Eu₂O₃ (purity: 99.9%) were prepared, and thecrystals had the crystal compositions of the TGG type polycrystals ofthe samples 25 to 27 shown in Table 2.

Then, the crystals of the samples 14 to 27 were cut into a disc shape,and mirror-finished by polishing with a high-precision polishing device.The Faraday rotor 2 was irradiated with He—Ne laser beams with an outputof 1 mW and a wavelength of 633 nm while a magnetic field (H) of 1 kOewas applied. Then, regarding the light beam outgoing from the Faradayrotor, the Faraday rotation angle (θ_(f)) was determined by the CrossNicol method. The value was substituted for θ_(f) of V=θ_(f)/(d×H).Thus, the effective Verdet constant was determined. Table 2 shows theresults. TABLE 2 Sample Crystal Verdet constant number composition (°/Oe· cm) *14 Tb_(2.995)Pr_(0.005)Ga₅O₁₂ 0.0077 15 Tb_(2.9)Pr_(0.1)Ga₅O₁₂0.0105 16 Tb_(2.8)Pr_(0.2)Ga₅O₁₂ 0.0135 17 Tb₂Pr₁Ga₅O₁₂ 0.0375 18Tb₁Pr₂Ga₅O₁₂ 0.0675 *19 Tb_(2.995)Ce_(0.005)Ga₅O₁₂ 0.0078 20Tb_(2.9)Ce_(0.1)Ga₅O₁₂ 0.0115 21 Tb_(2.85)Ce_(0.15)Ga₅O₁₂ 0.0140 22Tb_(2.3)Ce_(0.7)Ga₅O₁₂ 0.0355 23 Tb₂Ce₁Ga₅O₁₂ 0.0475 *24 Tb₃Ga₅O₁₂0.0075 *25 Tb_(2.8)Nd_(0.2) Ga₅O₁₂ 0.0090 *26 Tb_(2.8)Sm_(0.2)Ga₅O₁₂ —*27 Tb_(2.8)Eu_(0.2)Ga₅O₁₂ 0.0079

As seen in Table 2, the Verdet constants of the samples 25 to 27 inwhich at least one of cerium and praseodymium is not contained, and apart of the terbium is replaced by another additive are smaller comparedto the Verdet constant of the sample 24 which is a pure TGG singlecrystal. On the other hand, it is seen that the Verdet constants of thesamples 15 to 18 and the samples 20 to 23, which are within the scope ofthe present invention, are larger compared with the Verdet constant ofthe sample 24 which is the pure TAG single crystal. Regarding thesamples 14 and 19 in which the amounts of cerium and praseodymium areless than 0.01. Thus, it is seen that the effects of the cerium and thepraseodymium are not so sufficient to increase the Verdet constants. Thesample 26 absorbed a He—Ne laser beam with a wavelength of 633 μm, andthus, the determination was impossible.

[Experiment 3]

First, the single crystal of the sample 2 as a Faraday rotor, obtainedin Example 1 in which praseodymium was substituted for a part of theterbium, the single crystal of the sample 7, obtained in Example 2, inwhich cerium was substituted for a part of the terbium, and the puresingle crystal of the sample 10 obtained in Comparative Example 1 wereprepared. Each of the single crystals was processed into a columnarshape with a length of 1 mm and a diameter in cross-section of 1 mm. Theratio of the length from one end from the other end of the Faraday rotorto the diameter of the Faraday rotor was 1. Then, a polarizer and ananalyzer made of rutile and collective lenses made of quartz wereprepared. One of the collective lenses, the polarizer, the Faradayrotor, the analyzer, and the other collective lens were arranged in theoptical-axial direction in that order from the incident side of a lightbeam. Total reflecting mirrors were provided between a light irradiatingmeans and the collective lens and between a photodetector as alight-sensing means and the collective lens. The light irradiating meanswas disposed in such a position that a light beam irradiated by thelight irradiating means could be reflected on the total reflectingmirror at an angle of 90°, and the reflected light beam would becomeparallel to the Faraday rotor. Moreover, the light-sensing means wasprovided in such a position that a light beam outgoing from the Faradayrotor would be reflected on the total reflecting mirror at an angle 90°.Thus, magnetic sensors each having a configuration shown in FIG. 5 wasproduced using the samples 2, 7, and 10, respectively.

The variation of a magnetic field was determined by the method describedbelow. First, an impeller having magnets at the tops thereof wasdisposed on the side of the Faraday rotor of each magneto-opticalsensor. The impeller was rotated by means of a motor, so that themagnetic field intensity was in the range of 50 Oe to 250 Oe. FIG. 6illustrates the variation of the magnetic field. Then, eachmagneto-optical sensor was irradiated with a laser beam by means of aHe—Ne laser (wavelength: 633 nm). The change of the quantity of a lightbeam outgoing from the Faraday rotor with the variation of the magneticfield was determined by means of a photodetector. FIG. 7 shows thechange of the quantity of light.

As seen in FIGS. 6 and 7 regarding the magneto-optical sensors using theFaraday rotors made of the samples 2, 7, and 10, the curve of FIG. 6showing the magnetization-variation applied to the Faraday rotor and thecurves of FIG. 7 showing the light-quantities detected by thephotodetector have the same wave-form. This shows that all the variationof the magnetic field was converted to the light-quantity. Moreover,regarding the sample 2 in which praseodymium was substituted for a partof the terbium, and the sample 7 in which cerium was substituted for apart of the terbium, the difference between the detectedlight-quantities is larger compared to that in the sample 10 of the pureTAG single crystal. This shows that the magneto-optical sensors of thesamples 2 and 7 have a high sensitivity.

In the above-description, typically, the terbium type paramagneticgarnet single crystal of the present invention is used in amagneto-optical sensor for measurement of electric current. In addition,the terbium type paramagnetic garnet single crystal can be used as amagneto-optical sensor material for rotation sensors, flow-rate sensors,and so forth. Moreover, the terbium type paramagnetic garnet singlecrystal of the present invention can be used in magneto-optical devicessuch as optical isolators, optical attenuators, optical switches,optical circulators, and so forth.

Industrial Applicability

As described above, the terbium type garnet paramagnetic single crystaland the magneto-optical device of the present invention can be used invarious fields in which the change of situations must be sensed bydetection of a magnetic field with a light beam, e.g., the fields ofelectric power, motorcars, plants, and so forth, optical communicationfields in which light must be transmitted with stability.

1. A terbium type paramagnetic garnet single crystal containing at leastterbium and at least one element of aluminum and gallium, wherein atleast one element of cerium and praseodymium is substituted for a partof the terbium.
 2. The terbium type paramagnetic garnet single crystalaccording to claim 1, wherein the single crystal is represented by thechemical formula: (Tb_(3-x)M_(x))N₅O₁₂, in which M represents at leastone element of Ce and Pr, N represents at least one of Al and Ga, and xis in the range of 0.01≦x≦2.
 3. The terbium type paramagnetic garnetsingle crystal according to claim 2, wherein the single crystal isrepresented by the chemical formula: (Tb_(3-x)M_(x))N₅O₁₂, in which Mrepresents Ce, N represents at least one of Al and Ga, and x is in therange of 0.01≦x≦1.
 4. The terbium type paramagnetic garnet singlecrystal according to claim 2, wherein the single crystal is representedby the chemical formula: (Tb_(3-x)M_(x))N₅O₁₂, in which M represents Pr,N represents at least one of Al and Ga, and x is in the range of0.01≦x≦2.
 5. A magneto-optical device including, as a magneto-opticalelement, the terbium type paramagnetic garnet single crystal defined inclaim
 1. 6. A magneto-optical device including, as a magneto-opticalelement, the terbium type paramagnetic garnet single crystal defined inclaim
 2. 7. A magneto-optical device including, as a magneto-opticalelement, the terbium type paramagnetic garnet single crystal defined inclaim
 3. 8. A magneto-optical device including, as a magneto-opticalelement, the terbium type paramagnetic garnet single crystal defined inclaim 4.